Embodiments of the invention hereinafter described pertain to devices for the generation of Karman vortices.
Copending U.S. patent application Ser. No. 108,196 entitled Vortex Generating Device is filed on even date herewith, is commonly assigned, and is incorporated herein by reference. That copending application discloses various embodiments of vortex generating devices comprising a narrow generator plate immersed so that its axis of elongation is parallel to the direction of flow of a fluid stream. When fluid flow occurs over a smooth flat plate, such as the generator plate of the above-referenced copending application, viscosity causes the fluid velocity to be zero at points along the surface of the plate. However, at a very slight distance away from the surface of the plate the fluid velocity approaches a velocity generally characteristic of the fluid stream. Hence, a thin layer of fluid, known as the boundary-layer, containing large velocity gradients forms over the surface of the immersed plate.
The boundary layer initially starts at practically no thickness at a leading edge of the flat plate (defined with reference to the direction of fluid flow). Near the leading edge the flow of the boundary layer is essentially laminar. The boundary layer increases in thickness in a downstream direction as the viscous action increases due to increasing plate area. However, as the boundary layer becomes thicker and includes more fluid mass, instability results and flow within the layer breaks down, or "shears off", into turbulent flow. This change from a laminar to a turbulent boundary layer is not an abrupt change, but rather occurs through a boundary-layer transition region in which both viscous and turbulent action are present. Eventually the viscosity effects in the transition region are finally replaced by turbulent effects, and a wholly turbulent boundary layer results.
The particular points along the surface of the flat plate where the transition region occurs are related to a well-known parameter referred to as the Reynold's number. In this regard, the Reynold's number at any particular point downstream from the leading edge of the plate is dependent upon both the velocity of the fluid and the distance of that point from the leading edge. It has generally been observed that the transition region occurs for flat plates in a neighborhood approximating a Reynold's number of 10.sup.5. Thus, points along the surface of a flat plate having a Reynold's number in this neighborhood will generally be in the boundary-layer transition region.
The location of the boundary-layer transition region is not necessarily constant. That is, while a given flat plate may have a transition region associated with a first range of surface points for one velocity of fluid flow, the transition region may shift so as to become associated with a second range of surface points for a second velocity of fluid flow.
Regardless of where a boundary-layer transition region is located along the flat plate immersed in the stream of fluid flow, the transition region is attended by an erratic frictional drag. This frictional drag has a significant and, owing to its erratic nature, a generally unpredictable impact upon the fluid flow velocity in the transition region.
From the foregoing it is apparent that if a boundary-layer transition region were to occur near a sensing or shedding region of a device such as a vortex flowmeter, the erratic frictional drag would greatly disturb the rate and/or pattern of vortex formation with the result that measurements based thereon would be highly inaccurate. For example, if the structure of any particular vortex generating device were such that the boundary-layer transition region developed near a sensing region or shedding of the flowmeter for any fluid flow velocity of interest for a particular environment, the flowmeter measurements for that fluid velocity would be inaccurate.
Therefore, an object of embodiments of this invention is to provide a vortex generating device capable of producing a stable vortex street throughout a range of fluid flow velocities of interest for a given environment. Such embodiments advantageously eliminate the occurrence of the boundary-layer transition region at significant locations along the generating device where they would otherwise occur for the range of fluid flow velocities of interest.